The present invention relates generally to turbine airfoil components in gas turbine engines and particularly to increased oxidation resistance and/or abrasion resistance in squealer tips.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine, typically mounted or connected to the same shaft. The flow of gas through the rotating portion of the turbine comprising turbine blades turns the turbine, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine driving it forward. The hot exhaust gases flow past the turbine blades causing the blades to turn which in turn causes the shaft to turn and the engine to operate. However, some of the gases flow around the tips of the rotating blades, escaping from the engine between a static seal structure that encases the blades and the blade tips. The flow of gases through the gap between the seal structure and the blade tips decreases the efficiency of the engine. This gap can be caused by one or more of a variety of different factors. Thus, in order in increase the efficiency of the engine, it is desirable to eliminate or minimize the gap between the blade tip and the static seal.
The seal portions of a turbine that encase turbine blade tips generally consist of two principal elements, rotating turbine tips and a non-rotating, stationary seal on the stator. Rotating turbine tips extend radially outward from turbine blades toward the static seal and frequently have rows of thin tooth-like projections, commonly referred to as squealer tips. The static seal or stator is normally comprised of a mating metallic surface which may be a thin (filled or unfilled) honeycomb ribbon configuration or a solid surface such as a shroud with or without flow path cooling. These principal elements are generally situated circumferentially about the axial (lengthwise) dimension of the engine and are positioned with a small radial gap therebetween to permit assembly of the rotating turbine blades and static components.
The effectiveness of the turbine engine varies directly with the proportion of gas that impinges upon the turbine. One factor is fit-up of the blade tips to the seal. Closer tolerances between the rotating and static seals assists in achieving greater efficiencies. However, the fabrication process to obtain these close tolerances is extremely costly and time-consuming.
When the gas turbine engine is operated, the rotating seal can expand radially more than the stator, causing the squealer tip to rub into the stator seal, creating frictional contact between the squealer tip and the stator. This frictional contact in conjunction with the elevated operating engine temperature causes the squealer tip temperatures well in excess of 2,000 degrees F. with resulting possible damage to one or both seal members. The rub itself causes material typically to be removed from the squealer tip region of the blade. Such materials include the outermost portions of the blade which invariably include at least some or all of the protective environmental coatings that are usually applied to blades. As a result of the contact and the resultant high temperatures, squealer tips may crack, oxidize and recede, significantly impairing the seal efficiency and operation of the engine.
The shroud or seal construction is used to reduce the surface area on which the squealer tip rubs and helps to minimize the heat transferred into the rotating seal. In addition, blade tips and in particular squealer tips are made thin. However, excessive wear from deep rubs into a static seal, whether a solid shroud or filled honeycomb, can damage the rotating squealer tips, negatively affecting durability and engine efficiency. Furthermore, material transfer or removal can occur which also degrades the seal characteristics.
These temperature extremes, particularly those found in the gas path environment, or hot sections of the engine, contribute to the degradation of components, i.e. squealer tips, by the oxidizing and corrosive environments. Environmental coatings and thermal barrier coating (TBC) systems are often applied to the external surfaces of these components to protect the bare alloy from this hot environment. TBC systems also afford the opportunity to improve the efficiency of the engine by increasing operating temperatures. The oxidation-resistant coating systems are generally comprised of a metallic environmental coating (bond coat) applied to the structural component, and, in the case of TBC systems, an insulating ceramic layer applied on top of the bond coat.
Rubbing of the blade tip against the seal causes the removal of the insulating ceramic layer, if present, and the environmental coating from the tip region of the blades, allowing direct exposure of the less oxidation resistant substrate alloy to hot oxidizing gases. A consequence of this is more rapid oxidation of the squealer tip causing tip recession and cracking. The resulting increase in the clearance between the blade tips and shroud causes significant loss in engine efficiency and increase in engine operating temperatures as the gas temperatures are raised to achieve equivalent thrust, further exacerbating the problem.
Various coating techniques have been applied to the substrate metal in an attempt to increase both service life and operating efficiencies. For example, U.S. Pat. No. 5,603,603 to Benoit et al. is directed to applying by electrodeposition an abrasive tip coating to the blade tips, while U.S. Pat. No. 4,884,820 to Jackson et al. is directed to bonding a ceramic or metallic coating to the blade tips.
Industries other than the jet aircraft industry have also attempted to increase heat and abrasion resistance. For example, U.S. Pat. No. 4,060,250 to Davis et al. is directed to non-aircraft centrifugal compressors, in which the carbon steel rotary elements are inlaid or coated with a corrosion and heat resistant alloy, such as a chromium-containing nickel-based alloy. The surface of the rotatable cylindrical member is characterized by this metallurgically bonded fused alloy coating.
Other attempts include a metal-ceramic composite for use in a heating furnace disclosed in U.S. Pat. No. 4,947,924 to Morita et al. and an infiltration technique to improve the abrasion ability of the surface of a cutting tool such as a grinder by using a binder metal in a layer filled with grains of tungsten carbide, disclosed in U.S. Pat. No. 5,261,477 to Brunet et al.
Thus, there is a continuous need for improved designs for rotating turbine blade squealer tips to increase both service life and engine operating efficiencies. In particular, there remains a need to extend the service life of the squealer tip following the unavoidable loss of some or all of the squealer tip environmental coating. The present invention fulfills this need, and further provides related advantages.
The present invention provides for a squealer tip to include some proportion of a highly oxidation-resistant material, and a method for casting same, such that if any environmental coating were removed, the tip would retain some increased level of environmental resistance. The oxidation-resistant material optionally may also be a high abrasion resistance material, such that recession of the tip due to rubbing against a static seal would be reduced. The present invention also includes the squealer tip produced by the foregoing method.
An abrasion-resistant and/or oxidation-resistant material is placed and suitably anchored into the tip region of a wax precursor of a turbine airfoil mold prior to casting. Before casting and after coating the wax with a material that can withstand high temperatures to form a wax preform, the wax perform is enclosed within a mold, and wax is removed and drained during a low temperature heating cycle, leaving the oxidation-resistant material anchored at the tip region. During the casting operation, the abrasion-resistant and/or oxidation-resistant material that remains at the tip region is not completely melted, but remains anchored in the tip of the region even as the molten metal of the substrate alloy flows around the tip. As the alloy used to form the majority of the turbine blade solidifies, the abrasion and/or oxidation resistant material is incorporated into the turbine airfoil structure by the solidification of the alloy around it.
One advantage of the present invention is improved airfoil squealer tip performance and decreased squealer tip recession. Because the quality of the seal directly impacts engine efficiency, less recession results in less resultant decrease in sealing capability and hence, greater engine performance. The abrasion resistant alloy will survive the rubs against the adjacent static seal with less damage to the tip and less wear.
Another advantage is the improved temperature capability of high pressure turbine (HPT) airfoil tips, enabling higher T4.1 temperatures (first stage turbine-located immediately aft of the combustor) in the hottest engines, with resultant increase in engine efficiencies.
Yet another advantage is that existing investment casting operations can be easily modified to utilize the method of the present invention.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying figures which illustrate, by way of example, the principles of the invention.
FIG. 1 is a representation of a jet turbine engine turbine blade showing the squealer tip region.
FIG. 2 is a cross sectional representation of a squealer tip wax precursor with a monolithic material insert.
FIG. 3 is a cross sectional representation of a squealer tip wax precursor void in an investment mold with a material insert comprised of a plurality of small pieces.
FIG. 4 is a cross sectional representation of a squealer tip with material below the tip surface.
FIG. 5 is a cross sectional representation of a squealer tip, material and a TBC.